Immunothrombosis: A bibliometric analysis from 2003 to 2023

Background: Immunothrombosis is a physiological process that constitutes an intravascular innate immune response. Abnormal immunothrombosis can lead to thrombotic disorders. With the outbreak of COVID-19, there is increasing attention to the mechanisms of immunothrombosis and its critical role in thrombotic events, and a growing number of relevant research papers are emerging. This article employs bibliometrics to discuss the current status, hotspots, and trends in research of this field. Methods: Research papers relevant to immunothrombosis published from January 1, 2003, to May 29, 2023, were collected from the Web of Science Core Collection database. VOSviewer and the R package “Bibliometrix” were employed to analyze publication metrics, including the number of publications, authors, countries, institutions, journals, and keywords. The analysis generated visual results, and trends in research topics and hotspots were examined. Results: A total of 495 target papers were identified, originating from 58 countries and involving 3287 authors from 1011 research institutions. Eighty high-frequency keywords were classified into 5 clusters. The current key research topics in the field of immunothrombosis include platelets, inflammation, neutrophil extracellular traps, Von Willebrand factor, and the complement system. Research hotspots focus on the mechanisms and manifestations of immunothrombosis in COVID-19, as well as the discovery of novel treatment strategies targeting immunothrombosis in cardiovascular and cerebrovascular diseases. Conclusion: Bibliometric analysis summarizes the main achievements and development trends in research on immunothrombosis, offering readers a comprehensive understanding of the field and guiding future research directions.


Introduction
Immunothrombosis is a pathological process of intravascular innate immunity formed by the mutual interaction of immune cells and coagulation substances, considered as one of the physiological processes in host defense. [1]Key factors involved in immunothrombosis include platelets, [2] neutrophils, [3] the complement system, [4] and the coagulation factors. [5]Some infectious or noninfectious factors, upon activating the immune system, lead to the interaction of immune cells, especially neutrophils, with activated platelets. [6]Platelets can be activated either directly by the pathogen's stimulation or through the endothelial system or inflammatory response. [7]The interaction between platelets and leukocytes not only promotes the further recruitment of immune cells but also facilitates platelet adhesion and aggregation, triggering a cascade of coagulation reactions. [8]In addition, the involvement of neutrophil extracellular traps (NETs) and the complement system intensifies the entire process. [4,9]Immunothrombosis arises from the coordinated interaction of the immune system, complement system, and coagulation system.
Under normal circumstances, intravascular immunothrombosis is beneficial for capturing and clearing pathogens invading the bloodstream.However, uncontrolled immunothrombosis leads to pathological thrombus formation. [10]Dysregulated inflammatory states such as sepsis, [11] systemic lupus erythematosus (SLE), [12] acute respiratory distress syndrome, [13] stroke, [14] This study collected literature from the WOSCC database for analysis.The Web of Science is the most commonly used database for bibliometric analysis, and its recorded data is considered the most comprehensive and reliable. [21]The literature retrieval strategy for this study was "(((((TS = (Immunothrombosis)) OR TS = (Thromboinflammation)) OR TS = (Thromboinflammatory)) OR TS = (immunothrombus)) OR TS = (Immunothrombotic)) OR TS = (Thromboplasmin flammation)," with the index date set from January 1, 2003, to May 27, 2023.The document types selected were "Article," "Review Article," or "Early Access."After cleaning the initially retrieved literature data, articles with insufficient relevance to the research content in the field of immunothrombosis and duplicate publications were excluded, resulting in a total of 495 target papers.

Bibliometric analysis
Apply VOSviewer to gather basic information on authors, countries, institutions, sources, citations, keywords, and references from the target papers. [22]Utilize the Bibliometrix R package to compile information on the annual publication volume of target papers, distribution of authors' nationalities, core journals, and the evolution trend of keywords, along with thematic information. [23]

Results
A total of 686 publications in the past 20 years were preliminarily included, then 495 publications (284 articles and 211 reviews) were selected after screening.The 495 publications were published in 223 journals with 3287 authors from 1011 research institutions in 58 countries, citing a total of 27,940 references.

Publications
Over the past 2 decades, there has been an overall upward trend in the publication volume of papers within the field of immunothrombosis.Papers related to immunothrombosis were first published in 2005, but before 2015, the annual publication volume in this field was consistently below 10 papers per year.After 2015, there has been a steady increase in the annual publication volume, with a rapid surge observed from 2019 onwards, reaching the highest value in 2022.Figure 1 illustrates the temporal distribution of paper publication volume in the field of immunothrombosis.

Authors
According to the Price Law, [24] authors with no <3 papers were defined as the core authors in the field of immunothrombosis, resulting in a total of 141 core authors.Table 1 presents information on the top 5 most productive authors in terms of publication volume in this field.
To further clarify the collaboration among core authors, VOSviewer was used to generate a co-authorship knowledge map, illustrating their collaboration relationships as shown in Figure 2. The 141 core authors were categorized into 34 clusters, with 25 being independent clusters that did not collaborate with each other.The remaining 9 clusters collaborated, forming one large research group.Analysis of Figure 2A reveals that within this large research group, there are collaborative relationships among clusters represented by core authors such as Bo Nilsson, John D. Lambris, Konstantinos Ritis, Jason S. Knight, Steffen Massberg, Behnood Bikdeli, Bernhard Nieswandt, and Frederik Denorme.
Analysis of Figure 2B indicates that within the largest collaborative group, there are significant differences in the average publication time among various research teams.Clusters represented by Bo Nilsson and Bernhard Nieswandt had an average publication time before 2018, indicating early involvement in this field.Steffen Massberg individual average publication time is in 2018, but among the collaborators, the average publication time is later than 2020.This suggests that Steffen Massberg, as one of the early pioneers in the field, has facilitated the formation of newer research groups.Groups led by Tom Eirik Mollnes, Jason S. Knight, and Behnood Bikdeli have an average publication time after 2020, indicating later engagement in the field.
The research focus of Tom Eirik Mollnes' team (group 1) mainly involves the role of the complement system in promoting thrombosis and inflammation. [25,26]Behnood Bikdeli group (group 3) focuses on the clinicopathologic features of thrombosis in COVID-19 patients, [27] treatment strategies, [28] and the clinical application of anticoagulants in COVID-19 patients. [29]n addition, Behnood Bikdeli has an average citation per paper of 426.0, garnering widespread attention.
Among independent research groups, the group represented by Marie Ebeyer-Masotta (group 4) has the most recent average publication year, which is April 2022.Among the emerging research groups formed after 2020, the group represented by Jason S. Knight (group 2) has the highest total publication volume.[32] Dennis McGonagle (group 5) has an average publication year of 2021 and has contributed 8 papers to this field, with a total citation of 644.8]

Countries and institutions
This study conducted a statistical analysis of publication information for 58 countries involved in the field of immunothrombosis, visualizing and analyzing the collaboration among countries with more than 5 papers.Table 2 presents the top 5 high-productivity countries in terms of publication volume.The cooperation between countries with more than 5 papers was showed in Figure 3. Figure 3A illustrates the global distribution of the frequency of author affiliation appearances, with research institutions located in the United States appearing most frequently, followed by Germany and Italy.This suggests that researchers affiliated with institutions in these countries are more active in the field of immunothrombosis.Figure 3B shows the country-wise distribution based on the corresponding author's location.Among the top 10 countries in terms of   To better analyze the collaboration between countries, Figure 3C presents a co-occurrence map of countries with a total publication volume of at least 5 papers.The United States, the United Kingdom, Germany, and Italy engage in frequent collaborations with multiple countries, with U.S. researchers being the most active in participating in international collaborative research.Additionally, it can be seen that the top 3 countries in terms of average citation per paper, Greece, Australia, and Belgium, have 19, 16, and 13 collaborating countries, respectively (an additional file shows this in more detail, see Table S1, Supplemental Digital Content, http://links.lww.com/MD/N522).This information suggests that participating in multi-country collaborations is beneficial for enhancing research quality.
Analyzing the affiliations of authors can provide further insights into the background of research teams in the field of immunothrombosis.Table 3 compiles information on the top 11 institutions by publication volume.To better analyze the collaboration between research institutions, a co-occurrence knowledge graph was generated for institutions with a publication volume of at least 5 papers (Fig. 4).Among the 59 eligible institutions, 56 institutions have formed collaborative relationships.All of the top 11 most productive institutions are all prestigious traditional institutions in Europe and the United States.

Sources
This study analyzed 495 target papers published in 223 journals, with 36 journals having a publication volume of at least 3 papers, totaling 275 published articles, accounting for 55.6% of the total papers in this field.Table 4 displays the top 5 journals by publication volume, all of which belong to the Web of Science Journal Impact Factor Quartile 1 journals, with a focus on hematology.Figure 5 presents the basic information of core journals in the field of immunothrombosis.Using Bradford Law, bibliometrix identified the core journals in this field, as shown in Figure 5A.
Figure 5B illustrates the cumulative publication volume of the top 5 core journals from 2005 to 2023, with Frontiers in Immunology having the highest overall publication volume, while the other 4 core journals show no significant differences in publication volume.Figure 5C presents the annual average publication volume of core journals, highlighting that the Journal of Thrombosis and Haemostasis was the earliest to publish articles in this field (since 2005).Since 2018, Frontiers in Immunology has consistently had the highest annual publication volume.Notably, the International Journal of Molecular Sciences contributed mostly in 2022, making it a relatively new journal in the core group.

Keywords
This study conducted an analysis of the author keywords in 495 target papers, selecting keywords that appeared no fewer than 10 times (a total of 80).A keyword co-occurrence knowledge map was generated, as shown in Figure 6.Detailed information about keyword clusters can be found in Table 5, and information on the top 20 high-frequency keywords, ranked by occurrence frequency, is provided in Table 6.
High-frequency keywords summarize the research focuses in the field of immunothrombosis."COVID-19" is the most frequently appearing keyword."Thrombus formation" and "inflammation" are the physiological processes of immunothrombosis."Platelets" are the material basis for the process  of immunothrombosis."NETs" represent an important mechanism inducing thrombus formation."TF," " Von Willebrand factor (vWF)," "complement," and "P-selectin" are key molecules on immunothrombosis process.Analyzing the average publication years of keywords provides insights into the changing trends of research hotspots in the field of immunothrombosis.Figure 7 visualizes the relationship between the frequency of occurrence and the average publication year of these 80 keywords.Among the 80 high-frequency keywords, most have an average publication year around 2020. "P-selectin," "adhesion," and "in vivo" appeared earlier (in 2018 or earlier), while "endothelial dysfunction," "complications," and "plasminogen activator inhibitor-1 (PAI-1)" appeared later, around 2022, indicating emerging research hotspots.

Hotspots and frontiers
A thematic map (Fig. 8) was generated for the keywords, with the horizontal axis representing the relevance to the field and the vertical axis representing the development level of the theme.Thus, research topics represented by various keywords were divided into 4 quadrants.
Motor themes: These represent research topics with high relevance to the field and rapid development.In recent years, "complement system," "vWF," and "fibrinogen" have become key in exploring the mechanisms of immunothrombosis.The relationship between immunothrombosis and processes like "hemostasis," "VET," and "pulmonary embolism (PE)" is a major research focus.
Basic themes: These indicate topics with lower development but high relevance, serving as the foundation of research in the field.Research related to "thrombosis," "platelet," "inflammation," and "NETs" is fundamental, exploring the processes and mechanisms of immunothrombosis.Under the "COVID-19" theme, studies analyze the connection between the coagulation system and the immune system after COVID-19 infection from the perspective of immunothrombosis.
Emerging or declining themes: The topics in this quadrant represent either newly discovered or declining themes.There are 5 themes in this quadrant, these topics have seen fewer explorations in recent years.
Considering that the concept of "immunothrombosis" was first defined in papers published in 2013, and in light of the widespread outbreak of COVID-19 in 2019, leading to an in-depth investigation of the connection between COVID-19 and immunothrombosis, three-time intervals, namely 2005 to 2013, 2014 to 2018, and 2019 to 2023, were chosen to generate keywords Sankey diagram (Fig. 9).
As depicted in Figure 9, before the term "immunothrombosis" emerged, the research focus in this field was on "thrombosis" and "inflammation."From 2014 to 2018, the emphasis shifted towards "inflammation," and new keywords   such as "ischemic stroke," "acute coronary syndrome," and "middle cerebral artery occlusion" were introduced.This period reflected researchers' recognition of the role of immunothrombosis in cardiovascular and cerebrovascular diseases.Additionally, the appearance of "thrombin" and "platelet" indicated a growing interest in understanding the mechanisms of immunothrombosis.

General information
The annual publication volume in the field of immunothrombosis reflects the progress of this domain.Despite the search data covering the period from 2003 to 2023, there were no reported studies before 2005.From 2005 to 2013, the average annual publication volume in the field of immunothrombosis was 1.6 papers, with a total of 14 publications, indicating a relatively sparse research landscape in this area.The earliest published study discovered that platelet was an essential role in the cascade of inflammatory reactions. [39]During this period, research focused more on the interaction between platelets and leukocytes, with platelet-expressed Toll-like receptor (TLR) 2 and P-selectin participating in the processes of thrombosis and/or inflammatory reactions. [2,40]On the other hand, the binding of endothelial cell E-selectin to neutrophils led to the capture of platelets, causing vascular damage. [41]Before the concept of "immunothrombosis" was defined, researchers described this phenomenon as "Thromboinflammation" or "Thromboinflammatory." From 2014 to 2018, there was a significant increase in the number of publications in this field, with an average annual publication volume of 14.2 papers, indicating that the confirmation of the concept stimulated the development of related research in the field.In 2019, the annual publication volume was 44 papers, whereas the average annual publication volume increased to 91.5 papers after 2020.It is worth noting that among the 366 target publications published since 2020, a total of 289 papers are related to immune dysregulation and thrombosis, of which 180 papers focusing specifically on COVID-19, accounting for 62% (for more details, see Table S2, Supplemental Digital Content, http://links.lww.com/MD/N523).This indicates that a surge in annual publications of thrombosis application is closely related to the prevalence of COVID-19.
The collaboration among core authors reflects the level of activity among researchers in the field, and the formation of research teams also indicates the vitality of the field.All high-productivity authors in Table 1 belong to the large research group in Figure 2B.This suggests that close collaboration among core authors may promote the publication of articles and enhance research quality, accelerating the development of the field of immunothrombosis.Figure 2B showed that new research groups are gradually emerging and developing under the experienced researchers such as Steffen Massberg    In our analysis of high-productivity authors and institutions, we found that establishing collaboration between countries and institutions is beneficial for increasing both the quantity and quality of research papers.Countries such as the United States, the United Kingdom, and Germany not only hold the top 3 positions in terms of the number of papers but also maintain close international collaboration relationships between nations and institutions affiliated with these high-productivity Western countries.It is noteworthy that Harvard Medical School has achieved high citation numbers through multiple involvements in high-quality research rather than leading the research itself. [8]he journals publishing articles related to immunothrombosis are all classified as Journal Impact Factor Quartile 1 journals, meaning that publications related to immunothrombosis have been recognized by mainstream academic journals.Interestingly, there is an association between the proportion of open-access (OA) articles in journals and their publication and citation volumes.OA journals such as Frontiers in    significance of platelet pattern recognition receptors.The intravascular innate immune system consists of blood cascade systems (complement, coagulation, and fibrinolysis systems), blood cells (leukocytes, platelets), and endothelial cells.This intrinsic intravascular immune system mediates immune responses and thrombosis to purify the blood.Essentially, it protects the body, but dysregulation of the intravascular innate immune system leads to thromboinflammation. [5] Platelets play a crucial role in preventing inflammationrelated bleeding and protecting tissue cells from damage.Platelet-leukocyte interactions are at the core of intravascular innate immunity and contribute to thrombosis. [42]The TLRs on the surface of platelet membranes play essential roles in mediating platelet aggregation, platelet-leukocyte binding, NETs activation, and thrombin formation.TLR4, TLR1/2, and TLR9 can activate downstream signaling pathways through the PI3K/Akt pathway, participating in immunothrombosis. [43]Additionally, platelet-derived extracellular vesicles play a crucial role in recruiting leukocytes and facilitating intravascular immunothrombosis.Platelet-derived extracellular vesicles can serve as biomarkers for disease progression and severity. [44]urthermore, the complement system is an integral part of the intravascular innate immune system, serving as its backbone.Coagulation factors in the blood, including thrombin and FXa, can cleave complement Component 5 (C5), leading to the formation of complement C3a and C5a, as well as the membrane attack complex C5b-9.This process results in cell lysis and death. [45]2.2.Cluster 2: the relationship between NETs and immunothrombosis.In Cluster 2, the relevant research topics of the keywords includes mechanisms by which NETs induce immunothrombosis in different diseases and therapeutic targets related to NETs.NETs are DNA mesh structures modified with histones and granule proteins, representing a unique link between inflammation and thrombosis.Overall, the histones in the structure of NETs can stimulate platelet aggregation, while providing a scaffold for the adhesion of red blood cells.Simultaneously, NETs capture platelet adhesion molecules (vWF, fibrinogen, fibronectin) from the plasma, supporting the formation of thrombin-dependent fibrin, thereby stabilizing thrombosis.[46] In cardiovascular diseases, neutrophils aggregate in the vascular lesions, releasing NETs to promote the intravascular thrombosis.[47] In this process, key roles are played by peptidylarginine deiminase (PAD) 4, vWF, platelet TLR4, P-selectin, high mobility group protein 1, neutrophil solute carrier family 44 member 2, and TF.[48] In COVID-19, the live COVID-19 virus can stimulate the release of NETs from human neutrophils in a dosedependent manner through PAD4-mediated citrullination.[49] Simultaneously, COVID-19 triggers the activation of the complement system, where complement C3 activates platelets, NETs, and TF expression.[4] Therefore, there is a strong correlation between NETs and the severity of COVID-19 respiratory diseases.[50,51] The combined action of complement, platelets, NETs, and the TF/thrombin axis contributes to the formation and exacerbation of immunothrombosis within the pulmonary and renal microcirculation of patients, leading to organ failure.
Cluster 5: molecular mechanism of immunothrombosis.Cluster 5 illustrates the detailed mechanisms through which immunothrombosis induces various immune system diseases or contributes to disease development.Within the innate immune system, the blood vessels ensure their structural integrity and protection against pathogen invasion through coagulation.TF is a crucial initiator of the coagulation cascade, and the clot composed of fibrin serves as the scaffold for immunothrombosis.Erythrocytes act as bait to attract pathogens, while platelets, by initiating the complement system, releasing chemotactic factors, and expressing P-selectin, recruit leukocytes, activate FXII, and induce NETs, linking inflammation and coagulation.This process guides immune cells to the infected sites within the circulation. [1]It is noteworthy that TF plays a central role in triggering the process of immunothrombosis.Innate immune signals induce the expression of the TF gene in immune cells through NF-κB and activate stimulator of interferon genes and inflammasomes to trigger TF release, [95] driving the coagulation cascade. [96]Therefore, targeting TF is currently a widely studied therapeutic strategy.
APS is a typical immunothrombosis disorder.In vitro studies have found that antiphospholipid antibodies can promote the expression of TF, induce the release of pro-inflammatory cytokines, and trigger immunothrombosis by inducing NETs. [31]n sepsis, pathogens directly activate endothelial cells and platelets through their pathogen-associated molecular patterns, triggering NETs and leading to uncontrolled immunothrombosis in the microcirculation. [11]n acute respiratory distress syndrome, accompanying lung damage, platelets promote the recruitment of immune cells by expressing intracellular adhesion molecule-1, vascular cell adhesion molecule-1, and P-selectin.The formation of plateletneutrophil complexes activates triggering receptors expressed on myeloid cells-1, leading to the release of a large amount of pro-inflammatory cytokines and chemokines, further inducing immunothrombosis. [13]n respiratory diseases associated with RNA virus infections, immunothrombosis may be based on a positive feedback mechanism.Thrombin, through the activation of protease-activated receptor 1/2, maintains and amplifies the pro-thrombotic and pro-inflammatory effects exposed to viral RNA mimics like poly(I:C) on endothelial cells. [97]

Further development
The overall trend in the research topics related to immunothrombosis is illustrated in Figure 9.Over the past 20 years, researchers initially observed the correlation between thrombosis and inflammatory reactions in clinical cases, recognizing the crucial role of platelets in the immune system and the involvement of leukocytes in the process of thrombosis.After the introduction of the concept of "immunothrombosis," researchers shifted their focus to the role of immunothrombosis in cardiovascular and cerebrovascular diseases.
However, with the outbreak of COVID-19 in 2019, thromboembolic events in COVID-19 patients garnered unprecedented attention from researchers.Future research directions for researchers may be guided by the results in Figure 8.The primary research focus continues to center on the formation of immunothrombosis in COVID-19.In-depth molecular mechanism studies can still be conducted on NETs and platelets as crucial components of immunothrombosis.Additionally, the complement system, vWF, and fibrinogen are expected to become new hotspots for in-depth research, as their roles in immunothrombosis are not yet fully understood.In terms of related diseases, targeting immunothrombosis and developing clinical drug strategies for diseases such as COVID-19, APS, PE, coronary artery disease, and VET will be crucial research directions in the future.Besides, topics within niche themes may provide researchers with more perspectives.Research directions such as brain ischemia, middle cerebral artery occlusion, and neuroprotection are worthy of attention. [58,98,99]

Limitations
One limitation of this study is that all data were collected solely from WOSCC, potentially omitting articles from other databases such as PubMed and Scopus.Additionally, the number of published articles in the field of immunothrombosis increased significantly in 2023, with new research findings emerging rapidly.During the course of this study, the latest publications in this field were not included for analysis.These factors contribute to incomplete data collection, which may introduce certain limitations to the findings presented in this paper.

Conclusions
Immunothrombosis is a physiological process of mutual influence between the immune system and the coagulation system.It represents a form of innate immunity within blood vessels.This study employs bibliometric methods to analyze the development and frontier research topics in the field of immunothrombosis over the past 20 years.The number of research papers in this field is growing rapidly, with emerging research groups continuously contributing.
Research focus in the field of immunothrombosis has evolved from the initial study of the interaction between platelets and leukocytes to the current exploration of key components such as NETs, the complement system, and TF.Inhibiting inflammatory reactions has emerged as a potential new strategy for treating thromboembolic diseases.On the other hand, achieving a balance between immune responses and coagulation during disease treatment and modulating the extent of immunothrombosis have become critical considerations.Finding treatment targets that are more specific and controllable presents one of the current challenges in this field.

Figure 1 .
Figure 1.Number of annual publications in the field of immunothrombosis.

Figure 2 .
Figure 2. Authors' cooperation analysis.(A) Nodes with the same color belong to the same cluster, the size of nodes represents the publication volume, and the thickness of connecting lines indicates the number of collaborations.(B) Node size represents the total citation count for each author, node color indicates the average time between publications, with colors closer to yellow indicating more recent publications.The thickness of lines between nodes represents the number of collaborations between authors.

Figure 3 .
Figure 3. Analysis of the cooperation between countries.(A) Global distribution of the occurrence frequency of authors' institutions.(B) Multiple country publications and Single country publications number of corresponding authors in the top 10 most productive countries.(C) Network visualization map of countries cooperation.The size of the node represents the number of publications, the thickness of the link represents the amount of cooperation between 2 countries, and the color of the node represents average citation per paper of the country.

Figure 4 .
Figure 4. Network visualization map of institutions cooperation.

Figure 5 .
Figure 5. Core journals in the field of immunothrombosis.(A) Core journals identified by the Bradford law.(B) The cumulative publication volume of the top 5 core journals from 2005 to 2023.(C) The annual average publication volume of the top 5 core journals from 2005 to 2023.

Figure 6 .
Figure 6.Network visualization of author keywords.The color of the nodes represents the clusters to which the keywords belong, the size of the nodes represents the frequency of keyword occurrence, and the thickness of the lines connecting nodes indicates how frequently different keywords appear together in the same paper.

and
John D Lambris.Additionally, among the 25 independent research teams, 14 (64%) have an average publication time later than 2019.All these factors indicate that research on this field is in a continuous upward trend of development.

Figure 7 .
Figure 7. Overlay visualization of keywords average publication year.The size of nodes represents the frequency of keyword occurrence, the thickness of the lines connecting nodes indicates how frequently different keywords appear together in the same paper, and the color of nodes represents the average publication year of the keyword.

Figure 8 .
Figure 8.The thematic map of keywords.

Table 1
Publication information for high-productivity core authors.

Table 2
High-productivity country information.

Table 3
Information on research institution publication volume.

Table 4
Core journal information.

Table 5
Cluster information for keywords.